A137148 - OEIS

%I #15 Oct 26 2024 10:45:57

%S 1,8,12,32,54,40,48,84,120,128,108,160,252,220,192,500,312,486,336,

%T 240,512,660,544,840,432,684,936,640,504,880,1080,1012,768,2058,1000,

%U 1632,1248,972,2200,1344,2052,1624,960,1860,2268,2048,3120,1320,2176,3036

%N a(n) = k*phi(k), where k is the n-th nonprime number.

%C Numbers that occur in A002618 but not in A036689.

%H Amiram Eldar, <a href="/A137148/b137148.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A002618(A018252(n)). - _R. J. Mathar_, Jan 18 2021

%F Sum_{n>=1} 1/a(n) = A065484 - A136141 = 1.430699927388... . - _Amiram Eldar_, Oct 26 2024

%t a = {}; Do[If[!PrimeQ[n], AppendTo[a, n EulerPhi[n]]], {n, 1, 100}]; a

%Y Cf. A002618, A018252, A036689, A065484, A136141.

%K nonn,easy

%O 1,2

%A _Artur Jasinski_, Jan 23 2008